The present invention relates to a semiconductor device, and more particularly to a semiconductor device having a high carrier mobility.
It has been proposed to fabricate a field effect transistor (FET) using silicon or compound semi-conductive materials such as GaAs and AlGaAs in such a manner that a carrier transport channel layer is buried in a semiconductor layer. In order to form a buried channel layer, it is required to produce a potential well for confining carriers (namely, electrons or holes). As is well known, a potential well (that is, nonuniform distribution of inner potential) can be obtained by forming a heterostructure of semiconductor layers having different band structures, or by doping a semiconductor layer selectively with an impurity. An example of a buried channel due to a heterostructure is a GaAs channel layer sandwiched between GaAlAs layers which have a large band gap. In this case, electrons from doped GaAlAs layers are confined in an undoped GaAs channel layer (that is, potential well), and travel therein. Thus, a field effect transistor having such a heterostructure is usually called "double-heterostructure modulation-doped field effect transistor DH-MODFET (refer to the Japanese J. Appl. Phys. Vol. 23, No. 2, 1984, pages L61 to L63).
An example of a buried channel due to selective doping is an n-type impurity doped thin layer formed in silicon. In this case, a potential well is formed by dopant ions, and electrons are supplied from dopants. Thus, the thin doped layer can act as a channel layer. A field effect transistor having such a thin doped layer is usually called "doped-channel FET" (refer to the Japanese J. Appln. Phys. Vol. 26, No. 12, 1987, pages L1933 to L1936).
In the MODFET, no impurity ions are present in the channel layer. Accordingly, ionized impurity scattering does not occur, and a high carrier mobility can be obtained. While, in the doped-channel FET, the carrier concentration in the channel layer can be increased.
A field effect transistor using an Si.sub.0.2 Ge.sub.0.8 layer as a channel layer is disclosed on pages 308 to 310 of the IEEE Electron Device Lett., Vol. DEL-7, 1986. In this field effect transistor, as shown layer 320 are successively piled on a silicon substrate 1 through heteroepitaxial growth techniques, to form a two-dimensional hole gas at a p-Si/Si.sub.0.2 Ge.sub.0.8 interface. It has been confirmed that this field effect transistor operates as an MODFET. It is to be noted that the Si.sub.0.2 Ge.sub.0.8 channel layer 200 has a thickness of 250 .ANG. which is smaller than the critical thickness of a commensurately grown (under strained) Si.sub.0.2 Ge.sub.0.8 layer on the silicon substrate 1.
The MODFET using a GaAs layer as the channel layer, however, has a problem due to a material property of GaAs. That is, the electron mobility in GaAs is as high as 8,600 cm.sup.2 /V.sec at room temperature, but the hole mobility in GaAs is as low as 250 cm.sup.2 /V.sec at room temperature. When an n-MODFET and a p-MODFET, each provided with a GaAs channel layer are fabricated to form a complementary logic circuit, the large difference in mobility between electrons and a positive holes raises a problem. In the doped-channel FET, there arises the problem that electron mobility in the channel layer is reduced by impurity ion scattering therein by about one order of magnitude, as compared with the electron mobility in undoped silicon. In the MODFET of FIG. 1, the sheet carrier concentration in the channel layer is as low as 2.5.times.10.sup.11 cm.sup.-2, and the transconductance g.sub.m is as low as 2.5 mS/mm. This is because the band discontinuity .DELTA.E.sub.V at the Si/Si.sub.0.8 Ge.sub.0.2 interface is as small as 0.15 eV, and thus it is impossible to confine a sufficiently large number of positive holes in the potential well. In order to improve the characteristics of this MODFET, it is necessary to make the x-value of the Si.sub.1-x Ge.sub.x layer greater than or equal to 0.4, thereby making the band discontinuity .DELTA.E.sub.V greater than or equal to 0.3 eV. However, the critical thickness of the strained growth of an Si.sub.1-x Ge.sub.x layer having an x-value greater than or equal to 0.4, on the silicon substrate is less than 200 .ANG., so it is very difficult to form a channel layer having a sufficient thickness by strained growth.